Darwin : Mark the mod init/term section starts with a linker-visible sym.

Darwin : Mark the mod init/term section starts with a linker-visible sym.

Some newer assemblers emit section start temp symbols for mod init and term
sections if there is no suitable symbol present already.
The temp symbols are linker visible and therefore appear in the symbol tables.
Since the temp symbol number can vary when debug is enabled, that causes
compare-debug fails.  The solution is to provide a stable linker-visible
symbol.

Signed-off-by: Iain Sandoe <iain@sandoe.co.uk>
gcc/ChangeLog:

	* config/darwin.c (finalize_ctors): Add a section-start linker-
	visible symbol.
	(finalize_dtors): Likewise.
	* config/darwin.h (MIN_LD64_INIT_TERM_START_LABELS): New.

(cherry picked from commit 66526c6a1c01b6110eaeda94ecc885177c636605)